1. Field
The present invention relates generally to wireless communications, and more specifically to equalizers.
2. Background
For Code-Division Multiple-Access (CDMA) systems such as IS-856 (also referred to as High Data Rate (HDR)), cdma2000, and wideband CDMA (W-CDMA) it is desirable to implement a linear equalizer (LE) at the mobile terminal. The LE can mitigate the intersymbol interference (ISI) arising from multipath propagation and imperfect filtering on the forward link from the base station to the mobile terminal. A challenge for implementing a LE is determining the equalizer coefficients.
In the forward link of the HDR system, pilot symbols known to the receiver are transmitted at full power during a predetermined portion and interval of a transmission frame. This is referred to as a time-division-multiplexed (TDM) pilot. The receiver can tune to the pilot symbol interval. The algorithms that adapt the equalizer coefficients to their desired values are often based on the criteria of minimizing mean square error (MMSE) between the known pilot symbols and the equalizer's channel impulse responses of these pilot symbols. Two common examples of adaptive MMSE algorithms are the least-mean-square (LMS) algorithm and the recursive-least-squares (RLS) algorithm.
In the forward link of the cdma2000 system and the High Speed Packet Data Access (HSPDA) of W-CDMA, however, the pilot symbols are continuously transmitted on a pilot Walsh channel that is orthogonal to the Walsh channels being used for data transmission. This is referred to as a code-division-multiplexed (CDM) pilot. There is no specific interval when the pilot symbols are sent. This makes it difficult to determine the equalizer coefficients in CDM pilot systems. In CDM pilot systems, the convergence and tracking behavior of the LMS and RLS algorithms may suffer relative to TDM pilot systems.
Thus a need exists for improving the channel impulse response of the equalizer coefficients.